Dynamic fluid control system for immersion lithography

ABSTRACT

A dynamic fluid control system and method capable of reducing dynamic forces from the fluid on the last optical element ( 20 ) and substrate stage ( 14 ) caused by the motion of the immersion fluid. The system includes an imaging element ( 12 ) that defines an image and a stage ( 14 ) configured to support a substrate ( 16 ). An optical system ( 18 ) is provided to project the image defined by the imaging element onto the substrate. The optical system ( 18 ) includes a last optical element ( 20 ). A gap ( 22 ) filled with immersion fluid is provided between the substrate ( 16 ) and the last optical element ( 20 ). A dynamic force control system ( 34 ) is provided to maintain a substantially constant force on the last optical element and the stage ( 14 ) by compensating for dynamic changes of the immersion fluid caused by the motion of the immersion fluid through the gap and/or movement of the stage.

RELATED APPLICATIONS

This application claims priority on Provisional Application Ser. No.60/584,543 filed on Jul. 1, 2004 and entitled “Fluid Control System forImmersion Lithography Tool”. The contents of Provisional ApplicationSer. No. 60/584,543 are incorporated herein by reference for allpurposes.

BACKGROUND

1. Field of the Invention

The present invention relates to immersion lithography, and moreparticularly, to a dynamic fluid control system and method capable ofcompensating for dynamic changes in the forces exerted on the lastoptical element and stage by the immersion fluid caused by the motion ofthe immersion fluid and movements of the stage.

2. Related Art

A typical lithography machine includes a radiation source, an imagingelement defining an image pattern, an optical system, and a wafer stageto support and move the wafer. A radiation-sensitive material, such asresist, is coated onto the wafer surface prior to placement onto thewafer table. During operation, radiation energy from the radiationsource is used to project the image pattern defined by the imagingelement through the optical system onto the wafer. The optical systemtypically includes a number of lenses. The lens or optical elementclosest to the wafer is sometimes referred to as the “last” or “final”optical element.

The projection area during an exposure is typically much smaller thanthe wafer. The wafer therefore has to be moved relative to the opticalsystem to pattern the entire surface. In the semiconductor industry, twotypes of lithography machines are commonly used. With so-called “stepand repeat” machines, the entire image pattern is projected at once in asingle exposure onto a target area of the wafer. After the exposure, thewafer is moved or “stepped” in the x and/or y direction and a new targetarea is exposed. This step and repeat process is performed over and overuntil the entire wafer surface is exposed. With scanning typelithography machines, the target area is exposed in a continuous or“scanning” motion. The patterning element is moved in one directionwhile the wafer is moved in either the same or the opposite directionduring exposure. The wafer is then moved in the x and y direction to thenext scan target area. This process is repeated until all the desiredareas on the wafer have been exposed.

Immersion lithography systems use a layer of fluid that fills the gapbetween the final optical element of the optical assembly and the wafer.The fluid enhances the resolution of the system by enabling exposureswith numerical apertures (NA) greater than one, which is the theoreticallimit for conventional “dry” lithography. The fluid in the gap permitsthe exposure with radiation that would otherwise be completelyinternally reflected at the optical-air interface. With immersionlithography, numerical apertures as high as the index of refraction ofthe fluid are possible. Immersion also increases the depth of focus fora given NA, which is the tolerable error in the vertical position of thewafer, compared to a conventional lithography system. Immersionlithography thus has the ability to provide resolution down to 50nanometers or lower.

In immersion systems, the fluid essentially becomes part of the opticalsystem of the lithography tool. The optical properties of the fluidtherefore must be carefully controlled. The optical properties of thefluid are influenced by the composition of the fluid, temperature, theabsence or presence of gas bubbles, and out-gassing from the resist onthe wafer.

The pressure and forces exerted by the immersion fluid on the lastoptical element and wafer stage should be constant. This desired result,however, is very difficult to achieve for a number of reasons.

With immersion lithography, the fluid is constantly removed andreplenished. The removal of the fluid helps recover any contaminants andheat generated during exposure. Ideally, the amount of fluid beingsupplied should equal the amount being removed. A precise equilibrium,however, is difficult to achieve in practice. An uneven flow rate, whichmay result in a varying volume of fluid under the last optical element,may cause the forces and pressures acting on the last optical elementand wafer stage to be dynamic.

The movement of the wafer stage also creates dynamic forces on the lastoptical element due to the behavior of the immersion fluid. For example,when the wafer stage starts accelerating, the shape of the fluid at thefluid-air interface, sometimes called the meniscus, changes. Themeniscus tends to extend outward at the leading edge and pull-in at thetrailing edge of the movement. The change in the shape in the meniscuscreates a change in the static pressure exerted on the last opticalelement and stage by the immersion fluid.

The motion of the stage also creates waves in the immersion fluid. Thesewaves may cause the last optical element to oscillate up and down aswell as perturb the wafer stage. If the oscillations are still occurringduring an exposure due to the lingering effects of the waves, theaccuracy and image quality may be adversely affected.

Vertical adjustments of the wafer may also cause the volume of the gapbetween the last optical element and the wafer to change. The surface ofa wafer is not perfectly flat. Vertical adjustments are made by thewafer stage, depending on the surface topography of the wafer, tomaintain the distance between the last optical element and the exposurearea constant. The volume of the space between the wafer and lastoptical element changes when the wafer is moved up and down. As thevolume changes, the pressure and forces of the immersion fluid acting onboth the last optical element and the wafer stage also change.

The dynamic forces and pressures acting on the last optical elementcaused by the motion of the immersion fluid may cause the last opticalelement to become distorted and/or moved either up or down from itsideal position. As a result, the last optical element may be out offocus, resulting in a poor exposure. Similar forces acting on the waferstage may affect its performance as well.

At high stage speeds the meniscus can be perturbed to the point where itbreaks down, particularly at the leading edge. The breakdown ischaracterized by the escape and deposition of fluid droplets on thewafer where it emerges from the fluid. Such droplets are undesirable.They can entrap air, creating bubbles, when the wafer passes under theimmersed lens on a subsequent scan. Also if the droplets dry on thewafer, any contaminants in the droplet, for example residues dissolvedfrom the resist, remain deposited on the wafer.

A dynamic fluid control system and method capable of compensating fordynamic changes in the forces exerted on the last optical element andstage by the immersion fluid caused by the motion of the immersion fluidand movements of the stage is therefore needed.

SUMMARY

A dynamic fluid control system and method capable of reducing dynamicforces from the fluid on the last optical element and substrate stage,caused by the motion of the immersion fluid, is disclosed. The systemincludes an imaging element that defines an image and a stage configuredto support a substrate. An optical system is provided to project theimage defined by the imaging element onto the substrate. The opticalsystem includes a last optical element. A gap filled with immersionfluid is provided is between the substrate and the last optical element.A dynamic force control system is provided to maintain a substantiallyconstant force on the last optical element and stage by compensating fordynamic changes of the immersion fluid caused by the motion of theimmersion fluid through the gap and/or movement of the stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a lithography machine according to the presentinvention.

FIG. 2 is a diagram of a dynamic force control system used with theimmersion machine of the present invention.

FIG. 3A is a top-down diagram of the sensors and actuators arrangedaround the last optical element of the dynamic force control system ofthe present invention.

FIG. 3B is a top-down diagram of the dynamic force control system,showing the effects of stage motion on the fluid meniscus.

FIG. 3C is a side view diagram of the dynamic force control system,showing the effects of stage motion on the fluid meniscus.

FIG. 4 is a diagram of a sensor used in the dynamic force control systemaccording to one embodiment of the invention.

FIGS. 5A and 5B are diagrams of other sensors according to additionalembodiments of the invention.

FIG. 6 is a schematic illustrating the operation of the control elementaccording to one embodiment of the present invention.

FIG. 7 is another schematic illustrating operation of the controlelement according to another embodiment of the present invention.

FIGS. 8A and 8B are flow diagrams illustrating the sequence offabricating semiconductor wafers according to the present invention.

FIG. 9 is a schematic illustrating the determination of a controlalgorithm for fluid pressure control.

DETAILED DESCRIPTION

Referring to FIG. 1, an immersion apparatus is shown. The immersionapparatus 10 includes an imaging element 12 which defines an image, astage 14 configured to support a substrate 16, and an optical system 18configured to project the image defined by the imaging element 12 ontothe substrate 16. The optical system 18 includes a “last” or “final”optical element 20. A gap 22 is provided between the substrate 16 andthe last optical element 20. A fluid injection and removal element 24provides immersion fluid between the substrate 16 and the last opticalelement 20.

In one embodiment, the imaging element 12 is a reticle or mask. In otherembodiments, the imaging element is a programmable micro-mirror arraycapable of generating the image, such as described in U.S. Pat. Nos.5,296,891, 5,523,193, and PCT applications WO 98/38597 and 98/330096,all incorporated herein by reference. In one embodiment, the stage 14 isa fine stage that is supported by a coarse stage (not shown). The finestage is responsible for fine position adjustment of the substrate 16in, depending on the design, anywhere from one to six degrees of freedom(x, y, z, ⊖x, ⊖y and ⊖z). Similarly, the coarse stage is responsible formoving the substrate 16 on the fine stage 14 in one to six degrees offreedom. According to various embodiments, the fine stage 14 may besupported on the coarse stage by magnetic levitation, air bellows,pistons, vacuum, or springs, as are all well known in the art. In yetother embodiments, the fluid injection and removal element 24 is anozzle such as that described in PCT application No. PCT/US04/22915filed Jul. 16, 2004 entitled “Apparatus and Method for Providing Fluidin Immersion Lithography” or the environmental system described in PCTApplication PCT/US2004/010055 filed Mar. 29, 2004 and entitled“Environmental System Including Vacuum Scavenge For ImmersionLithography Apparatus”, both incorporated by reference herein for allpurposes.

Referring to FIG. 2, a diagram of a dynamic force control system 30 usedwith the immersion machine 10 is shown. The system 30 includes one ormore pressure sensors 32 and one or more actuators 34 arranged adjacentto the last optical element 20 (for the sake of simplicity, only asingle sensor 32 and actuator 34 pair is shown in FIG. 2). The pressuresensors 32 are positioned adjacent the gap 22 in approximately the sameplane as the bottom surface of the last optical element 20. The bottomsurface of the last optical element 20 is sometimes referred to as the“boundary” surface of the lens because it bounds or is in contact withthe immersion fluid in the gap 22. The last optical element 20 is heldin position by a housing 38 of the optical system 18.

The system 30 also includes a control element 36. During operation, thesensors 32 measure pressure changes on the boundary surface of the lastoptical element 20. The control element 36 generates control signalsthat control the actuators 34 in response to the measured pressurereadings respectively. The actuators 34 create local changes in fluidpressure to compensate for the dynamic pressure changes caused by motionof the immersion fluid. For example, if the fluid pressure increases,the actuators act to relieve the pressure, and vice-versa. In oneembodiment, the sensors 32 and actuators 34 are arranged on the housing38 adjacent to and around the periphery of the boundary surface of thelast optical element 20. In another embodiment fluid flow sensors arealso used to help define the fluid dynamic state, as described in moredetail below.

Referring to FIG. 3A, a top-down view of the sensors 32 and actuators 34arranged around the last optical element 20 is shown. In the figure, thesubstrate 16 is shown positioned on the stage 14. The last opticalelement 20 is positioned over the substrate 16 and defines an imagingfield 40. The sensors 32 and the actuators 34 are arranged adjacent toand around the periphery of the last optical element 20. The fluidinjection and removal element 24 provides and removes the immersionfluid to and from the gap 22 (not visible in this view) between thesubstrate 16 and the last optical element 20.

The normal flow of the immersion fluid through the gap 22 creates staticforces on the last optical element 20 and stage 14. Changes in the flowrate of the immersion fluid, stage acceleration and motion, verticaladjustments of the wafer, etc., however, may all cause the immersionfluid to create dynamic forces on the last optical element 20 and waferstage 14. Sensors 32 positioned locally near or under the last opticalelement 20 monitor the local static and dynamic pressure changes andprovide information to the control element 36, so corrective measurescan be taken. According to one embodiment, the pressure sensors 32 arepositioned in the same horizontal plane as the boundary surface of thelast optical element 20. The pressure sensors 32 are oriented such thatonly the pressure normal to the surface of the boundary surface ismeasured. Since the immersion fluid is bounded by the horizontal planedefined by the boundary surface, there is no component of momentum inthe direction normal to the boundary surface.

FIGS. 3B and 3C show the effects of stage motion on the fluid meniscus21. In the figures the stage is moving to the right as designated byarrows 38. This motion causes the shape of the fluid boundary to bedifferent at the leading and trailing edges, as shown at 23 a and 23 brespectively. Specifically, the meniscus tends to extend outward at theleading edge 23 a and pull-in at the trailing edge 23 b. This dynamicchange of the immersion fluid creates dynamic changes in force on boththe stage and the last optical element. In addition, these changescreate waves 23 c in the fluid which propagate along the meniscus aroundthe lens region. These waves 23 c also contribute to the dynamicpressure changes on the last optical element 20 and the wafer stage.

The pressure sensors 32 used with the system 30 may be a manometer, acapacitive manometer, a piezoelectric transducer or any other type ofpressure sensor. The actuators 34 may be pistons, diaphragms, bellows,pressure head partial vacuum tubes, or electrocapillary pressureelements, such as described in M. Prins et al, Science 291, 277(2001),incorporated by reference herein for all purposes.

In other embodiments fluid flow sensors are also used to help define thefluid dynamic state. Referring to FIG. 4, a fluid flow velocity sensor50 according to one embodiment is shown. The sensor 50 is an “X-type”hot wire sensor that includes two non-contacting wires 52 a and 52 b ofrelatively short length that are mounted horizontally and at rightangles with the flow direction of the immersion fluid. During operation,the temperature of the two wires 52 a and 52 b are monitored. From themeasured temperature changes, the velocity of the immersion fluid in thenormal direction can be monitored. Examples of X-type hot wire sensorsare from TSI Inc. of Minneapolis, Minn. From the velocity measurements,in addition to the pressure measurements, the effects of changes in theimmersion fluid momentum and transient forces acting on the last opticalelement and wafer stage can be determined.

Referring to FIG. 5A, a diagram of another sensor 60 that can be used tomeasure both fluid pressure and velocity is shown. This sensor 60includes a total head tube 62 with a wall static pressure tap 64, suchas one of the pressure sensors mentioned above, to measure both thestagnation pressure (p_(o)) and the static pressure (p). The stagnationpressure is the pressure measured by a pressure sensor at a point wherethe fluid is not moving relative to the sensor.

Alternatively, as shown in FIG. 5B, a Pitot tube 66 is used to measureboth static and stagnation pressures. The pressure measured at point 68is the stagnation pressure (p_(o)) since the velocity of the local flowat the entrance of the tube is zero. The pressure (p) at point 70 isdifferent because the local flow velocity is not zero. The velocity ofthe fluid can then be calculation using Bernoulli's equation and thefollowing assumptions.p _(o) =p+½ρv ²,  (1)where ρ is the fluid density. The fluid velocity can then be determined:v=[2(p _(o) −p)/ρ]^(1/2)  (2)

Both local fluid flow velocity and pressure are thus determined withthis type of sensor.

There are several flow assumptions that restrict the use of Bernoulli'sequation:

1. steady flow

2. incompressible flow

3. frictionless flow (low viscosity)

4. flow along a streamline

Assumption 2 is assumed to be acceptable, because the flow velocitiesare much less than the speed of sound in the fluid. With assumption 4,it is assumed that the Pitot tube axis is aligned with the flowdirection. Since the fluid, by design, will typically flow along theaxis of the scanning stage, assumption 4 is acceptable. Assumption 3 isequivalent to requiring a high Reynolds number (but not too high forlaminar flow to be maintained). Assumption 1 however is questionable.Therefore calibration will be required for accurate velocity andpressure determination. The Pitot tube is also limited in frequencyresponse. If higher frequency response is desired, the hot wire velocitysensor may be used instead.

In unsteady flow, where streamline directions are changing, multiplePitot tube heads, pointing in orthogonal directions, or directions wherethe flow is known from past measurements to point, may aid in operationof the sensor.

Referring to FIG. 6, a schematic 60 illustrating the operation of thecontrol element 36 is shown. For clarity, only a single pressure sensor32 (such as the pressure and/or velocity sensors of FIGS. 3A, 5A or 5B),and actuator 34 are used. The pressure sensor 32 generates a pressurecontrol signal to the controller 36 that is indicative of the following:(i) the stage controller 62 directing the stage 14 to move in thehorizontal and/or vertical directions, the resulting stage motioncausing pressure change δPs at the pressure sensor 32; (ii) operation ofthe fluid injection/removal system 63 may cause an additional pressurechange δPf at the pressure sensor; (iii) simultaneous operation of theactuator 34 causes an additional pressure change δPa; and (iv) thestatic pressure P₀ as measured by the sensor 32. The total pressure asmeasured at the sensor 32 is therefore P₀+δPs+δPf+δPa. Using analgorithm, the controller 36 uses information from the pressure sensor32, along with information from the stage controller to generate acontrol signal 64 to the actuator 34. The actuator's response to thesignal 64 is a pressure change δPa at the pressure sensor 32 whichsubstantially cancels the pressure changes from the stage motion andfluid injection/removal system as specified by equation (3) below:δPa=−(δPs+δPf).  (3)

Thus the effect of dynamic pressure changes on both the last opticalelement 22 and the wafer stage are minimized. In other embodimentssignals from the velocity sensor or the stage controller may be absent,or information from the fluid injection/removal system may be providedto the controller.

In reducing the pressure fluctuations affecting the last lens elementand wafer stage, the controller 36 is likely to also reduce somewhat theamplitudes of the waves 23 c. This in turn may improve the performanceof the fluid injection/removal system. It may also reduce the chances ofbreakdown of the leading edge meniscus and thus avoid the formation ofisolated fluid droplets on the wafer.

The above description is appropriate for a linear system, where thepressure change δPs created by the stage motion is independent of thepressure change δPf created by the fluid injection/removal system, andthe pressure change δPa from the actuator. In reality, the fluid motionmay make the system response non-linear, so that δPs, δPf and δPa arefunctions of one another. However Eq. 3 remains valid. Also, if thepressure changes are small enough, the system response may beapproximated as linear.

Satisfying Eq. 3 is complicated by the fact that the controller can'trespond instantaneously to the pressure sensor signal, nor can theseparate contributions to the pressure sensor signal, δPs, δPf, δPa, bemeasured. Additionally, since fluid is moving, the rate of change of thepressures will be important as well. The controller therefore needs analgorithm to use information from the stage and fluid injection/removalsystems, as well as the total pressure signal, over a period of time toestimate the appropriate signal to send to the pressure actuator. Thealgorithm may be obtained in a number of ways:

1. A fluid dynamic model may be constructed of the fluid cell and thefluid dynamic forces associated with stage motion and fluid injection orremoval calculated. Pressure changes at the pressure sensor resultingfrom these effects are then calculated, resulting in an estimate of therequired pressure actuator signal. The model may have some adjustableparameters, whose setting will minimize the total pressure change at thepressure sensor, Eq. 3.

2. The algorithm may be established empirically, using an adaptivefilter to create a model of the fluid cell and its response to stage andfluid injection/removal perturbations. FIG. 9 illustrates such a filterand its training process. An adaptive filter is a linear filter withadjustable weights which are altered to make the output signal agreewith a desired signal. In FIG. 9 signals from the stage 14 and fluidinjection/removal element 24 also go to the adaptive filter as inputs.The output of the adaptive filter 92 is an estimate of the pressure atthe pressure sensor 32 caused by the actuator. The relation between theactuator signal and the pressure at the sensor 32 is established in anearlier calibration, in the absence of perturbations from the stage andfluid injection/removal systems. In FIG. 9 the desired signal is thesignal from the pressure sensor 32 caused by perturbations from thestage 14 and fluid the injection/removal element 24. The error signal sbetween the pressure sensor signal and the pressure sensor signalpredicted by the adaptive filter 92 is fed back to the filter and theweights adjusted to minimize ε. Note that the polarities of the summingjunction lead to the relation ε=δPs+δPf+δPa. Thus if the weights can besuccessfully adjusted to make δ negligibly small, we have establishedthe condition of Eq. 3.

After successfully training the adaptive filter 92, the controller 36containing the adaptive filter is connected to the system as in FIG. 6.

3. Adaptive filters are most appropriate for systems which are linear oronly weakly non-linear. If the fluid dynamics of the fluid cell couplethe pressure changes caused by the stage and fluid injection/removalsystems and the actuator together too strongly, the adaptive filter maybe replaced with a neural network system, which can represent non-linearrelations. The neural network is trained and utilized essentially thesame way as the adaptive filter.

If the environmental conditions of the fluid cell change, the optimalparameters of the controller algorithm may change as well. Thecontroller may include an adaptive feature which allows it to continueto train the algorithm, as environmental conditions change. Thus, if thealgorithm is based on a fluid dynamic model, certain adjustableparameters in the model may be changed to minimize the total pressurechange at the pressure sensor. If the algorithm is an adaptive filter ora neural network, the adjustable weights may be changed to minimize thetotal pressure change at the pressure sensor.

Referring to FIG. 7, another schematic illustrating operation of thecontrol element 36 coupled to multiple pressure and/or flow velocitysensors 32 and multiple actuators 34 is shown. In FIG. 3A, the number ofactuators was equal to the number of pressure and velocity sensors. Inalternative embodiments, however, the number of actuators 34, pressuresensors 32, and/or flow velocity sensors may be different. In FIG. 7there are n actuators 34 a-34 n, m pressure sensors 32 a-32 m, and kflow velocity sensors V1-Vk. Each sensor 32 a-32 m generates a pressuresignal P1-Pm derived from the four pressure components (i) through (iv)as discussed above. Each actuator 34 a-34 n creates a pressure change ateach of the pressure sensors 32 a-32 m, so that the measured δPa at eachpressure sensor is the accumulation of the pressure changes caused byall the actuators 34 a-34 n. More specifically, actuator i generates apressure change δPaij at pressure sensor j. The total pressure change atsensor j from all the n actuators is given by Σ_(i=1, n) δPaij. Thecontroller 36 processes the m pressure sensor signals, the k flowvelocity sensor signals, and information from the stage controller andfluid injection/removal system, and generates actuator signals a1-an tothe actuators 34 a-34 n, so that the actuator pressures at the pressuresensors satisfy the relations:Σ_(i=1,n) δPaij=−(δPsj +δPfj), for j=1,m.  (4)

This insures that the effects of dynamic pressure changes on both thelast optical element and the wafer stage are minimized. In other words,controlling the actuators 34 a-34 n enables the dynamic net forces andnet moments (i.e., torque) acting on the final optical element 20 andstage caused by the dynamics of the immersion fluid to be minimized.When the stage is moving, the contact angle of the immersion fluid isdifferent at the leading edge versus the trailing edge. This createsdifferent forces acting on the leading edge and trailing edges ofhousing 38 and the last optical element 20. These different forces maycreate net moments or torques on the last optical element or waferstage, which can be corrected using the aforementioned equation.

In one embodiment, as illustrated in FIG. 3B, the plurality of sensors32 and actuators 34 are arranged around the periphery of the lastoptical element 20. With this arrangement, the waves 23 c created at themeniscus of the immersion fluid are controlled, and breakdown of theleading edge meniscus during stage motion is avoided or minimized. Inother embodiments, the sensors 32 and actuators 34 can be arranged atdifferent locations adjacent the last optical element 20.

Designing an algorithm to satisfy Eqs. 4 is analogous to the descriptionabove in connection with FIG. 6 and Eq. 3.

Throughout this discussion, the terms force and pressure have been usedinterchangeably. It should be noted, however, that technically, the twoterms are slightly different. Pressure is a measure of force per unitarea. Many of the sensors that are commercially available are designedto measure pressure. Sensors, however, could be calibrated to measureforce and could be used with the present invention.

In normal operation the actuator signals will typically lie within alimited range of values, which are determined by the limited range offluid perturbations allowed by the controller 36. However if the fluidsystem is strongly perturbed, some actuator signals may fall outside theabove range. For example, if the fluid injection fails to completelyfill the gap 22, leaving an air void under the part of the last opticalelement, the actuator signals predicted by the controller 36 are likelyto differ substantially from their normal values. Or if the leading edgemeniscus breaks down, leaving isolated droplets on the wafer, air may bedrawn into the gap 22, and the actuator values predicted by thecontroller 36 may depart from normal values. It may not be possible forthe controller to recover from such pathological conditions, but theaberrant actuator signals can serve as a message to the lithography toolcontroller that proper immersion conditions in the gap 22 have beenlost, and lithographic exposure must be halted until the condition iscorrected.

Semiconductor devices can be fabricated using the above describedsystems, by the process shown generally in FIG. 8A. In step 801 thedevice's function and performance characteristics are designed. Next, instep 802, a mask (reticle) having a pattern is designed according to theprevious designing step, and in a parallel step 803 a wafer is made froma silicon material. The mask pattern designed in step 802 is exposedonto the wafer from step 803 in step 804 by a photolithography systemdescribed hereinabove in accordance with the present invention. In step805 the semiconductor device is assembled (including the dicing process,bonding process and packaging process), finally, the device is theninspected in step 806.

FIG. 8B illustrates a detailed flowchart example of the above-mentionedstep 804 in the case of fabricating semiconductor devices. In FIG. 6B,in step 811 (oxidation step), the wafer surface is oxidized. In step 812(CVD step), an insulation film is formed on the wafer surface. In step813 (electrode formation step), electrodes are formed on the wafer byvapor deposition. In step 814 (ion implantation step), ions areimplanted in the wafer. The above mentioned steps 811-814 form thepreprocessing steps for wafers during wafer processing, and selection ismade at each step according to processing requirements.

It should be noted that the particular embodiments described herein aremerely illustrative and should not be construed as limiting. Forexample, the substrate described herein does not necessarily have to bea semiconductor wafer. It could also be a flat panel used for makingflat panel displays. Rather, the true scope of the invention isdetermined by the scope of the accompanying claims.

1. An apparatus, comprising: an imaging element which defines an image;a stage that supports a substrate; an optical system that projects theimage defined by the imaging element onto the substrate, the opticalsystem having a last optical element; a gap between the substrate andthe last optical element, the gap being filled with an immersion fluid;and a dynamic force control system, the dynamic force control systemmaintains a substantially constant force on the last optical element andstage by compensating for dynamic changes of the immersion fluid causedby motion of the immersion fluid through the gap and/or movement of thestage.
 2. The apparatus of claim 1, wherein the dynamic force controlsystem further comprises a sensor positioned adjacent the last opticalelement.
 3. The apparatus of claim 2, wherein the sensor is positionedin the gap adjacent the last optical element.
 4. The apparatus of claim2, wherein the last optical element defines a boundary surface thatbounds the immersion fluid in the gap, the sensor positioned in the sameplane as the boundary surface.
 5. The apparatus of claim 2, wherein thesensor is at least one pressure sensor.
 6. The apparatus of claim 5,wherein the last optical element has a boundary surface that bounds theimmersion fluid in the gap, the pressure sensor measures pressurechanges substantially normal to the boundary surface caused by motion ofthe immersion fluid in the gap.
 7. The apparatus of claim 2, wherein thesensor comprises one from the following group of sensors: a manometer, acapacitance manometer, or a piezoelectric transducer.
 8. The apparatusof claim 2, wherein the sensor is at least one fluid flow velocitysensor.
 9. The apparatus of claim 8, wherein the fluid flow velocitysensor comprises two wires that measure the velocity of the immersionfluid in a direction normal to the surface of the last optical elementby measuring the temperature of the two wires.
 10. The apparatus ofclaim 8, wherein the fluid flow velocity sensor comprises a lead tubewith a wall static tap.
 11. The apparatus of claim claim 8, wherein thefluid flow velocity sensor comprises a Pitot lead tube with staticpressure holes.
 12. The apparatus of claim 2, wherein the sensormeasures static pressure changes created by the immersion fluid.
 13. Theapparatus of claim 2, wherein the sensor measures dynamic pressurechanges created by the immersion fluid.
 14. The apparatus of claim 1,further comprising a plurality of sensors each positioned adjacent thelast optical element.
 15. The apparatus of claim 1, wherein the dynamicforce control system further comprises an actuator that compensates forchanges in the force exerted on the last optical element and stagecaused by motion of the immersion fluid through the gap and/or motion ofthe stage.
 16. The apparatus of claim 1, wherein the dynamic forcecontrol system further comprises a plurality of actuators each arrangedat different locations adjacent a surface of the last optical element incontact with the immersion fluid.
 17. The apparatus of claim 15, whereinthe actuator comprises one from the following group of actuators:pistons, diaphragms, bellows, pressure head partial vacuum tubes, orelectrocapillary pressure elements.
 18. The apparatus of claim 1,wherein the dynamic force control system further comprises: a sensorpositioned adjacent a surface of the last optical element in contactwith the immersion fluid; an actuator located adjacent the last opticalelement, the actuator compensates for dynamic forces exerted on the lastoptical element and the stage; and controller that controls the actuatorin response to the sensor to compensate for the dynamic forces exertedon the last optical element and the stage caused by motion of theimmersion fluid through the gap and/or motion of the stage as measuredby the sensor.
 19. The apparatus of claim 18, wherein the controlleruses an adaptive filter algorithm to control the actuator.
 20. Theapparatus of claim 18, wherein the sensor generates a pressure signalindicative of one or more of the following components: (i) sensedpressure changes exerted by the immersion fluid caused by movements ofthe stage; (ii) sensed pressure changes exerted by the immersion fluidcaused by injection and removal of the immersion fluid; (iii) sensedpressure changes exerted by the immersion fluid caused by the actuator;and (iv) sensed static pressure exerted by the immersion fluid.
 21. Theapparatus of claim 20, wherein the controller generates a control signalto control the actuator in response to the pressure signal from thesensor and a fluid velocity signal.
 22. The apparatus of claim 1,wherein the dynamic force control system further comprises: a pluralityof actuators arranged adjacent the last optical element and in contactwith the immersion fluid, the plurality of actuators compensate fordynamic changes of the force exerted by the immersion fluid caused bymotion of the immersion fluid through the gap and/or motion of thestage; a plurality of velocity sensors that measure the velocity of theimmersion fluid; a plurality of sensors arranged adjacent the lastoptical element and in contact with the immersion fluid; each of theplurality of sensors generates a pressure signal indicative of thefollowing components: (i) sensed pressure changes exerted by theimmersion fluid caused by movements of the stage; (ii) sensed pressurechanges exerted by the immersion fluid caused by injection and removalof the immersion fluid; (iii) sensed pressure changes exerted by theimmersion fluid caused by each of the plurality of actuators; and (iv)sensed static pressure exerted by the immersion fluid; and a controllerthat generates a plurality of control signals to the plurality ofactuators respectively, the controller generating the plurality ofcontrol signals in response to the plurality of pressure signals fromthe plurality of sensors respectively.
 23. The apparatus of claim 22,wherein the plurality of sensors and the plurality of actuators arearranged around the periphery of the last optical element of the opticalsystem.
 24. The apparatus of claim 18, wherein the controller generatesa control signal to control the actuator, the control signal beinggenerated from an algorithm that uses information from one or more ofthe following: (i) movement of the stage; (ii) a fluid injection andremoval element that injects and removes immersion fluid from the gap;(iii) a pressure signal as measured by the sensor.
 25. The apparatus ofclaim 24, wherein the controller further uses the algorithm to predictthe control signal as learned over a period of time.
 26. The apparatusof claim 25, wherein the algorithm uses an adaptive filter to create amodel of the behavior of the immersion fluid in response to the motionof the stage and fluid injection and removal.
 27. The apparatus of claim25, wherein the algorithm uses a mathematical model that models thebehavior of the immersion fluid in response to the motion of the stageand the fluid injection and removal to predict the control signal. 28.The apparatus of claim 25, wherein the algorithm uses a neural networkthat models the behavior of the immersion fluid in response to themotion of the stage and the fluid injection and removal to predict thecontrol signal.
 29. The apparatus of claim 25, wherein the algorithmfurther includes an adaptive feature which adapts the algorithm tocompensate for environmental conditions changing at the sensor.